Field of the Invention
The present invention relates to load bearing pavements and their construction. More particularly, this invention pertains to an improved high quality asphalt pavement.
Description of the Prior Art
High quality asphalt pavements find many important uses. They are employed, for example, for highways that carry high volume auto and heavy truck traffic, airport runways and taxiways that service high volume, heavily loaded high density aircraft traffic and in port construction with regard to the transport, storage and transfer of containerized freight.
As used herein, high quality asphalt pavement refers to those pavements that are constructed primarily of high quality construction materials that may generally be obtained only by central plant manufacturing processes and that are placed with specialized construction lay down equipment. This assures that the various pavement materials are properly and uniformly densified, pavement layers are to proper line, grade, and thicknesses and that the upper-most layer provides a smooth riding surface that can safely support high speed vehicle traffic.
Asphalt concrete pavements are classified as flexible pavements as opposed to rigid or Portland cement concrete pavements. The two primary flexible pavement types are layered and full depth asphalt pavement. The full depth asphalt pavement comprises only dense-graded asphalt concrete placed directly on the subgrade. In layered asphalt pavements the highest quality materials are placed in layers nearest the surface. These materials, in the order in which they would probably exist in structural sections, beginning at the subgrade, include soil, pit run gravels, processed gravels, lime and/or cement treated soil and/or gravels, crushed rock and asphalt concrete. Parameters such as the stabilometer value and gravel equivalency factor are numerical measures of quality although in recent years, it has been recognized that asphalt concrete possesses some of the characteristics of a structural slab.
Both empirical and mechanistic methods are presently employed for the design of flexible pavements. Index parameters are often used to describe pavement materials, subgrade characteristics and traffic. Pavement systems generally arranged in accordance with prior art design philosophy and including variations of the above-referenced designs are shown in U.S. Ser. Nos. 936,493 of Travilla, 984,801 of Davis, U.S. Pat. Nos. 2,083,900 of Ebberts, et al., and 3,044,373 of Sommer.
Empirical design methods relate traffic to pavement performance commonly utilizing either a design equation or a series of design charts that relate thickness of the pavement section to projected traffic and strength of the reconstituted subgrade soil. Equivalent material thickness factors are employed to allow substitution of materials of the structural section. The equivalency factors employed vary with the particular design method. However, in general, a 40 to 60 percent reduction in thickness is realized when dense graded asphalt concrete is substituted for aggregate base.
An early empirical design technique is the stabilometer design procedure developed by the State of California and utilized in several of the Western states. A more recently developed empirical method is the AASHO Flexible Pavement Design Method of the American Association of State Highway Officials.
The mechanistic design of pavements is in part founded in fundamental mechanics and based upon well recognized analysis techniques. In mechanistic design the stress and strain fields within the pavement system are identified and the materials of the pavement section characterized. The characterization to be appropriate must reflect the influences of temperature and load rate on asphalt concrete stiffness and fatigue life, stress state on aggregate base and open graded asphalt concrete stiffness, and stress state and moisture content on stiffness and permanent deformation of the subgrade soils.
The identifications of the stress and strain fields are normally accomplished with the aid of elastic layered computer codes that incorporate elastic constants compatible with load rate, temperature, stress state and moisture content. Iterative techniques may be employed to reflect the influence of stress state on elastic constants. Computer analysis of the temperature and moisture fields can aid in the selection of elastic constants that appropriately reflect such environmental factors.
The evaluation of the mechanical design is accomplished by comparisons of the projected strains at critical locations (i.e. depths) of the structural section to predetermined materials failure criteria. While displaying an insight into certain significant mechanical characteristics of commonly employed pavement construction materials and their responses to loading, the prior art has failed to utilize such knowledge to derive optimum systems (i.e. pavement structures) based upon the stress-bearing capacities of conventional materials and thus the powerful mechanistic analytical techniques have not previously produced conceptually new and optimum pavement designs.